What forces contribute to a runner's forward propulsion?

[DeNovo]

Homework Statement

Homework Equations

None. Only free-body diagrams I think, but I'm unsure how to exactyl draw it.

The Attempt at a Solution

So, my reasoning is that it is D, but I'm not exactly sure. When drawing a free body diagram, I only draw one force of friction, but I think intuitively there would be two (C and D), why not both? Also, wouldn't A and B also exist, so why don't we have two FNs in the diagram? I'm confused.

 

[phinds]

Forces move the thing they are exerted on. How could a force that is not exerted on the person move the person?

 

[DeNovo]

Forces move the thing they are exerted on. How could a force that is not exerted on the person move the person?

I understand that, but why are all the free-body diagrams I do in class only include the force of friction going one way, such as this:

 

[phinds]

I understand that

Then why did you say "So, my reasoning is that it is D"

 

[phinds]

why are all the free-body diagrams I do in class only include the force of friction going one way]

Because you are doing them wrong.

 

[DeNovo]

So, I think it is "D" because A and B are upwards (perpendicular to surface) so wouldn't propel forward, and C would only slow the person/object down because it is in the opposite direction, not propel, so then just by process of elimination I think it is D and it seems to make sense because if I picture a foot hitting the ground "backwards" that seems to push the object forwards.

 

[phinds]

So, I think it is "D" because A and B are upwards (perpendicular to surface) so wouldn't propel forward, and C would only slow the person/object down because it is in the opposite direction, not propel, so then just by process of elimination I think it is D and it seems to make sense because if I picture a foot hitting the ground "backwards" that seems to push the object forwards.

I specifically told you that a force moves that on which it is exerted. You said you understand that, but you continue to insist that a force that is not acting on the person causes the person to move.

Do you understand Newton's Third Law? How does it apply in this case?

 

[DeNovo]

Sorry for my elementary physics skills, but I understand now that it is "C" because since a force moves that on which it is exerted, C is exerted on the person therefore that must be the force that pushes it.

 

[phinds]

Right and do you now understand why your force diagram is deficient?

 

[DeNovo]

Right and do you now understand why your force diagram is deficient?

It is deficient because of Newton's Third Law (force every force there must be an opposite and equal force). So, in the force diagram I would remove the FN and replace it with Ff(k) representing the force of the ground onto the runner.

 

[phinds]

You have two Fn's. I suggest you draw a new, neater and correct, diagram so we can be sure you have it right

 

[DeNovo]

 

[phinds]

Why do you feel that FFK1 and FFK2 are not equal and opposite? I ask you again, do you understand Newton's 3rd Law?

 

[DeNovo]

If I were accelerating, how would it look differently? What extra and unequal force is being added? That's the only reason I put ma.

 

[phinds]

If I were accelerating, how would it look differently? What extra and unequal force is being added? That's the only reason I put ma.

Rather than ask new questions, how about you answer MY question?

 

[DeNovo]

"do you understand Newton's 3rd Law?"

Yes, every force and an equal and opposite force, so when subtracting them it should be 0. But that's exactly why I'm confused how I would account for acceleration in the force diagram so that the right minus left forces would equal ma. Sorry for my inexperience.

 

[phinds]

The Earth pushes on the runner and the runner goes forward. The runner pushes on the Earth and the Earth goes backward. It's as simple as that.

 

[haruspex]

View attachment 209199

The best way to draw free body diagrams is to have a separate one for each body. In the present case, one for the person/foot, not showing the ground it rests on, and, optionally, a separate one for the ground. That helps avoid confusion over which force acts on what.
In the FBD for the foot you would show gravity, the normal force from the ground, and the forward-pointing frictional force.
If you have an FBD for the ground it would show the downward normal force from the person and the backward-pointing frictional force. And for completeness I suppose it should have an upward force, the gravitational attraction the person exerts on the Earth.

 

[DeNovo]

/ch

The best way to draw free body diagrams is to have a separate one for each body. In the present case, one for the person/foot, not showing the ground it rests on, and, optionally, a separate one for the ground. That helps avoid confusion over which force acts on what.
In the FBD for the foot you would show gravity, the normal force from the ground, and the forward-pointing frictional force.
If you have an FBD for the ground it would show the downward normal force from the person and the backward-pointing frictional force. And for completeness I suppose it should have an upward force, the gravitational attraction the person exerts on the Earth.

That makes it easier to understand, but how would the FBD look different if say when I was running I was speeding up than if I was running at a constant speed?

 

[haruspex]

That makes it easier to understand, but how would the FBD look different if say when I was running I was speeding up than if I was running at a constant speed?

The question as stated asked about forward propulsion, so I took this to be the accelerating case.
We can make it a bit more accurate by adding an opposing horizontal vector for drag. If running at constant speed what would the net horizontal force be?